(1) Field of the Invention
The invention relates to a system for signalling that overvoltage protecting means in the connecting circuits of telephone lines which are connected to the central battery through rest supply resistors and to a loop detector for detecting the state of the subscriber's loop and to switching means for connecting the telephone line to a feeding bridge are not intact.
It is necessary to protect telephone lines which are connected to telephone exchanges from overvoltages. Various means are known for this purpose inter alia fuses and clamping diodes are used.
In addition it is necessary to check that the fuses and the diode protection are intact.
(2) Description of the Prior Art
French Patent No. 2,223,918 discloses a system for detecting the interruption of a fuse in a telephone line to which a detector for false calls is added.
In this known system the fuse is constructed such that after excitation a short-circuit is formed between the wires of the telephone line. This short-circuit can be detected by means of the detector for false calls.